This invention relates to the measurement of fluid flow volume and, more particularly, to compensating such measurements for fluid density variations.
For measuring residential gas volume, gas utilities require that the residential meter be reliable, have high accuracy and be economical. In the past, the meter of choice has been the diaphragm meter, which is encased in a relatively large and unsightly enclosure. It has been proposed to replace the diaphragm meter with the wave meter as the next generation standard residential gas meter. Such a meter is disclosed in U.S. Reissue Pat. No. RE35,114, to Ingman, the contents of which are incorporated by reference herein.
As disclosed in the Ingman patent, such a meter includes a flexible membrane mounted in a housing in the fluid flow path such that the pair of faces of the membrane simultaneously define, with the housing, fluid impermeable seals at at least two different locations along the flow path. The membrane flexes so as to permit passage of discrete quantities of fluid, each having a known volume. The meter generates electrical signals corresponding to flexure of the membrane. By monitoring the frequency of these signals, the fluid flow rate is determined and utilized for calculating overall flow volume. However, for any given flow rate, the frequency of the AC voltage generated by the meter changes if the density of the gas changes. This has been referred to as a "density effect". This density effect is more pronounced at higher flow rates. The density effect between air and typical natural gas has been demonstrated to be approximately 7% at higher flow rates.
In the past, wave meters have been calibrated during manufacture using air. To account for the density effect, two calibration tables are utilized--one for air and one for gas. The table for gas is offset to compensate for the density difference. A major problem arises because the density of natural gas is not constant and, in fact, can vary as much as .+-.20% from its nominal value. These density changes cause the meter calibration to change and therefore cause a measurement error. It is therefore a primary object of the present invention to provide an arrangement for compensating wave meter volume measurements for gas density variations.